The present invention relates to ion-sensitive, or ion-selective, electrodes, and particularly to electrodes of the "membrane" type wherein a conductor extends into an electrode enclosure, or membrane, which has an ion-sensitive exterior immersed in the sample to be tested.
The primary intended use is as a pH sensor, wherein the exterior of the membrane is electrically sensitive to hydrogen ions. However, the invention also has utility with respect to electrodes which are sensitive to other types of ions, such as sodium ions or potassium ions. And sensors using the present invention may be used to measure such values as sulfate activity or halide activity.
The invention is primarily concerned with the problem of providing a suitable material for conducting the electrical signals generated at the membrane to the conductor which transmits the signals to an analyzer, which translates them into the desired data. In a pH sensor, the sensing of hydrogen ions at a measuring electrode, or half cell, is compared with data from a reference electrode, or half cell, to provide pH readings.
The conventional method of conducting electrical signals within the electrode has been the use of an aqueous buffered electrolyte filling the space between the membrane and the externally connected conductor. There are, however, certain obvious shortcomings resulting from the use of liquid electrolytes, such as the problems of "attitude-sensitivity" and of unfavorable response to significant temperature changes. The attitude-sensitivity problem requires complex sealing efforts to prevent leakage and, in addition to the leakage problem, also may cause a loss of electrical contact if the electrode is tipped. Furthermore, significant temperature increases can cause boiling of the liquid, with the attendant pressure buildup leading to destruction of the electrode.
One effort to solve the problem of attitudesensitivity has been the use of a gelled electrolyte in the electrode. In such a device, the membrane, or bulb, is totally filled with a gelled electrolytic material, which makes it attitude independent. However, in addition to the fact that gelled electrolytes may liquefy due to temperature increases, these gelled electrolytes suffer from their tendency to "poison" the pH response on the inner side of the pH responsive bulb, their susceptibility to degradation over a long period of time, and their possible instability in an ionizing environment, such as might be encountered in a field of radioactive flux.
Another prospective solution of the problems encountered by liquid-filled electrodes has been to provide a completely "solid-state" electrode. An exmple of this is shown in FIGS. 1 and 2 of Petersen et al. U.S. Pat. NO. 3,649,506, issued Mar. 14, 1972. In the device of the patent, the pH sensitive glass is melted and deposited as the outside layer on an electrode having a plurality of solid material layers. Devices of this type have experienced difficulties due to drift of their asymmetry (resting) potential. It is speculated that this drift may be caused by the different thermal coefficients of expansion of the pH sensitive glass and the solid material on which it is deposited. In other words, there may be a stability problem at the interface between the two materials. Another theory is that the pH sensitive glass, as it is deposited on the electrode, and as it changes from the molten state to the solid state, may tend to become devitrified and to lose the amorphous characteristic of glass, which would be detrimental to its functioning as a pH sensor.
The purpose of the present invention is to provide a more satisfactory solid-state ion-sensitive electrode, which will avoid the shortcomings of the devices discussed above.